imported MP3 unable to pan

My a cappella choir uses commercially available learning tracks, on which one channel will contain the music for one of the 4 sections, and the other channel will contain the music for the other sections. Typically the ‘part predominant’ section will be in the left channel, with the other ‘background’ sections in the right channel. At times, I need to split these stereo channels, and create a custom learning track. When split, I am not able to adjust the pan in the single track; I must export the track to an AIFF file, then re-import it. At that point I am able to pan the track to the other channel. Why can I not pan the MP3 file??

If you “split to mono”, both tracks will be center-panned (will sound “in the middle”) but you can still use the track’s “Pan” slider to pan the track left / right.


Thank you